Thrombopoietin potentiates collagen receptor signaling in platelets through a phosphatidylinositol 3-kinase-dependent pathway

Collagen activates platelets through a tyrosine kinase-dependent pathway, i nvolving phospholipase C gamma 2, Functional responses such as aggregation and secretion induced by collagen are potentiated by preincubation with thr ombopoietin (TPO), In this study, we show that collagen and thrombopoietin activate the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and that t his contributes to their respective actions. The structurally distinct Inhi bitors of PI 3-kinase, wortmannin, and LY294002, completely inhibit formati on of phosphatidylinositol 3,4,5-tris-phosphate by collagen, This leads to a substantial reduction in the formation of inositol phosphates and phospha tidic acid, 2 indices of PLC activity, and the consequent inhibition of int racellular Ca++ [Ca++](l), aggregation and secretion. Potentiation of the c ollagen response by TPO is prevented in the presence of wortmannin and LY29 4002, However, when the 2 PI 3-kinase inhibitors are given after the additi on of TPO but before the collagen, recovery of potentiation is observed. Th is suggests that potentiation is mediated through activation of PI 3-kinase , TPO stimulates aggregation of platelets from a low percentage of donors a nd this is also blocked by wortmannin, These results suggest that the PI 3- kinase pathway plays an important role in signaling by collagen and in the priming action of TPO. (C) 2000 by The American Society of Hematology.